1. Field of the Invention
The present invention relates to a residual capacity indicating device suitable for indicating the residual capacity of a secondary battery, such as those consisting of lithium cells.
2. Description of the Prior Art
Recently, many secondary batteries are used as a power supply for compact, portable electronic apparatus including radios, tape recorders and portable VTRs. The applicant of the present patent application proposed previously a residual capacity indicating device which indicates the residual capacity of a secondary battery by a plurality of LEDs in Japanese Patent Laid-open (Kokai) No. Hei 1-100479. This residual capacity indicating device will be described briefly with reference to FIG. 3.
In FIG. 3, indicated at 1 is a secondary battery such as that consisting of lithium cells. The lithium cell has a positive electrode of LiMn.sub.2 O.sub.4 or the like. The discharge voltage of the lithium cell drops with discharge time and the capacity of the same increases with temperature. The secondary battery 1 is connected to a portable electronic apparatus (which will be designated simply as "load"), not shown, and a residual capacity indicating circuit shown in FIG. 3 is connected to the secondary battery 1. The positive electrode of the secondary battery 1 is grounded through a variable resistor 2. The slider 2a of the variable resistor 2 is connected to the noninverting terminals of the differential amplifiers forming comparators 5a, 5b and 5c. The positive electrode of the secondary battery 1 is connected through resistors 3a, 3b and 3c to the inverting terminals of the comparators 5a, 5b and 5c. The respective inverting terminals of the comparators 5a, 5b and 5c are grounded through resistors 4a, 4b and 4c, respectively. The resistors 3a and 4a, the resistors 3b and 4b and the resistors 3c and 4c determine reference voltages to be applied to the inverting terminals, i.e., reference input terminals, of the comparators 5a, 5b and 5c, respectively. The resistances of the resistors 3a and 4a, the resistors 3b and 4b and the resistors 3c and 4c are determined according to detection points, respectively. The comparators 5a, 5b and 5c compare the voltage of the slider 2a of the variable resistor 2 connected to the secondary battery 1 with the reference voltages determined by the resistors 3a and 4a, the resistors 3b and 4b and the resistors 3c and 4c, respectively, and provides output signals if the voltage of the secondary battery 1 is higher than the reference voltages. The output terminals of the comparators 5a, 5b and 5c are connected to the bases of npn transistors 6a, 6b and 6c, respectively. The collectors of the transistors 6a, 6b and 6c are connected to the cathodes of LEDs 7a, 7b and 7c, respectively and the anodes of the LEDs 7a, 7b and 7c are connected to a supply terminal 8 to which a positive dc voltage is applied. The emitters of the transistors 6a, 6b and 6c are grounded. When the npn transistors 6a, 6b and 6c are in an ON state, the LEDs 7a, 7b and 7c connected to the collectors of the transistors 6a, 6b and 6c emit light. Suppose that the secondary battery 1 has discharge characteristics as shown in FIG. 4 showing the variation of discharge voltage with discharge time, and that the detection voltage V.sub.1, V.sub.2 and V.sub.3 are to be detected at three detection points A.sub.1, A.sub.2 and A.sub.3 corresponding to residual discharge times 0 min, 60 min and 120 min and compared with the reference voltages by the comparators 5a, 5b and 5c. Then, none of the LEDs 7a, 7b and 7c becomes radiant if the discharge voltage of the secondary battery 1 is lower than the voltage V.sub.1 at the detection point A.sub.1. If the discharge voltage of the secondary battery 1 is not lower than the voltage V.sub.3 at the detection point A.sub.3, all the LEDs 7a, 7b and 7c become radiant. If the discharge voltage of the secondary battery 1 is equal to the voltage V.sub.2 at the detection point A.sub.2, only the LEDs 7b and 7c become radiant. Thus, the residual discharge time of the secondary battery 1 can be estimated from the light emitting conditions of the LEDs.
The residual capacity indicating device described with reference to FIGS. 3 and 4 enables the estimation of the residual capacity of the secondary battery, which is very convenient in using the electronic apparatus powered by the secondary battery. However, the reference voltages applied to the reference voltage input terminals of the comparators 5a, 5b and 5c determined by the resistors 3a and 4a, the resistors 3b and 4b and the resistors 3c and 4c remain constant only while the second battery 1 is maintained at a constant temperature, for example, at a constant ordinary temperature. If the secondary battery 1 consists of lithium cells, the possible discharge time of the battery 1 varies with the ambient temperature. For example, the capacity of the secondary battery 1 increases with the increase of temperature and hence the residual discharge time increases. Furthermore, since the reference voltages rise with the increase of temperature, it is possible that only the LED 7c becomes radiant when the discharge voltage is V.sub.2 at the detection point A.sub.2 (FIG. 4) where both the LEDs 7b and 7c are expected to be radiant. Thus, the residual capacity indicating device is unable to indicate the actual residual capacity of the secondary battery 1 if the ambient temperature is variable.